1. Field of the Invention
The present invention is in the field of capacitor manufacture and relates more particularly to a method of manufacturing capacitors comprised of alternate layers of conductive metal electrodes and an interposed layer or layers of dielectric such as silicon dioxide.
2. The Prior Art
Conventional monolithic ceramic capacitors, and particularly capacitors having values within small tolerance ranges are extremely expensive to manufacure. The expense inheres in the manufacturing procedures and materials which have heretofore been required to be employed.
In the typical manufacture of a ceramic monolithic capacitor there is first formed a suspension of ceramic particles in an organic binder which enables the green ceramic materials to be extruded into thin pliant layers. In a succeeding step the layers are imprinted with electrode material as by silk screening procedures with a conductive ink having included metallic particles. The selected metal must be capable of withstanding the high heats of firing of the green ceramic without vaporization, thus mandating the use of platinum, paladium or like extremely costly noble metals.
Suitably imprinted layers of the green ceramic are stacked in such manner that the patterns of imprinted electrode material on alternate layers are disposed substantially in registry, but with edge portions of the electrodes on one layer slightly offset from the edge portions of the electrodes on the alternate layer. The process is repeated until a suitable stack of layers is formed, following which the stack is severed along cutting lines to form individual chips whereby there is exposed at opposite severance lines electrodes of one polarity only.
The resultant articles are thereafter heat treated first to vaporize the organic binder material and thereafter subjected to higher temperatures which function to fire the ceramic components and form a monolithic capacitor.
Thereafter the fired capacitors are terminated by applying a silver containing paste to the exposed electrode portions which treated edge portions are thereafter heated to effect a conductive relationship between each of the individual electrodes and the associated terminations.
The difficulties and high costs inherent in the outlined manufacturing steps are well known to those skilled in the art. These difficulties are multiplied when it is desired to produce capacitors to close tolerances. More particularly, it will be apparent that the capacitive value of each individual chip is dependent not only on the composition and thickness of the dielectric and the area of imprintation of the silk screened electrode material, but also on the precision with which adjacent layers bearing the imprintations are oriented one to the other.
In addition, careful controls must be exercised in respect of the volitilization and firing steps to produce a capacitor having values within a predictable tolerance range.
For close tolerance applications, it is conventional to manufacture capacitors in such manner that the capacitance exceeds that which is desired. After the capacitor is formed the overlap area of the electrodes is erroded or worn-away as by a sand blasting jet directed normal to the broad surface of the capacitor, so as to reduce the capacitance to the desired value. The net result of the manufacturing complexities outlined above may often raise the cost of a close tolerance capacitor to a multiple of a hundred or more times the cost of a capacitor of equal electrical properties, but not requiring close tolerance controls.
Numerous procedures have been suggested as a means of simplifying the manufacture of close tolerance ceramic or metal oxide capacitors of the so-called thin film type. By way of example U.S. Pat. No. 3,149,393 discloses forming a capacitor by depositing a silicon film on a substrate, oxidizing the silicon, applying metal to the oxidized silicon and thereafter forming a multilayered capacitor from the noted base material.
U.S. Pat. No. 3,679,942 suggests the formation of a thin film capacitor by depositing over a first conductive material a layer of silicon dioxide which is thereafter densified under wet gaseous atmosphere following which the oxide layer is dried, and a second electrode deposited. No means of terminating the resultant capacitor is suggested.
U.S. Pat. No. 3,469,294 pertains to a method of forming capacitors which includes electrolytically depositing a dielectric oxide layer on a surface of anodizable aluminum strip, forming an insulating grid over a pattern of interconnecting areas on the surface of the oxide layer, electrolytically depositing semiconductor oxides into the grid openings to form electrodes and thereafter severing the thus formed structure along the grid lines. Connections are effected to the resultant capacitors for purposes of termination, by forming silver deposits on the respective electrodes and connecting wires to the silver deposits.
U.S. Pat. No. 3,457,614 teaches forming a capacitor by depositing a anodizable metal on an insulating substrate, such as glass, anodizing the surface of the deposited metal to provide a dielectric film, depositing counterelectrodes, depositing copper lands on the ends of the counterelectrodes and thereafter dicing or subdividing the substrate along predetermined zonal lines to form individual capacitor units.
U.S. Pat. No. 3,591,905 is cited in view of its showing of a means of subdividing an enlarged capacitor forming sheet into a multiplicity of discrete capacitor units.
U.S. Pat. No. 3,809,973 discloses a termination method which involves placing a silver and thereafter a gold covering over the exposed electrode edges at the margins of the ceramic capacitor.
While various of the above noted references suggest means for the formation of thin film capacitors, none of the noted references enables the economical fabrication of compact thin film capacitors capable of being formed within precise tolerances ranges and capable of being terminated without elaborate soldering techniques and without the use of relatively high concentrations of noble metals.